Subminiature circuit protectors are useful in applications in which size and space limitations are important, for example, on circuit boards for electronic equipment, for denser packing and miniaturization of electronic circuits. Ceramic chip type fuses are known, but current structures are limited in size reduction by the structure of the fusing elements and encapsulation and sealing.
A problem in miniaturizing circuit protectors is that the small size of the fuse element and of the circuit protector itself results in a small contact area between the fuse element and the electrical terminations. The small contact area results in unnecessarily high resistance at the contact, and reduces the reliability and operation of the unit.
The present invention, generally, provides a subminiature surface mountable circuit protector that is simple and relatively inexpensive to manufacture. The present invention also provides a subminiature board mountable circuit protector that has improved reliability and operation.
The subminiature circuit protector of the present invention can be easily manufactured for a variety of voltage and current ratings.
The circuit protector according to the invention includes a substrate of electrically insulating material, such as ceramic or glass. The substrate has a flat top surface, opposing end edges and opposing lateral edges. Termination pads of electrically conductive material are deposited on the top surface at each end, and extend to the end edge and along a portion of the lateral edges. A fuse element of predetermined fusible response positioned across a space between the termination pads connects the termination pads to form a conductive path from end to end of the substrate. A cover of electrically insulating material suffuses over the termination pads and the fuse element to contact and envelop all of the underlying elements.
According to the invention, end terminations are formed by a coating of at least one layer of electrically conductive material that contacts the termination pads along the end edge and lateral edges of the substrate. The end terminations provide a greater contact area than previously known in the art for improving the electrical connection of the terminations to the fuse. In a preferred embodiment, the end coating comprises an inner layer of a silver alloy, a middle layer of nickel, and an outer layer of a tin/lead alloy. The end coating also extends along the lateral edges of the substrate as far as permitted by industry standards.
According to the invention, a cover may comprise glass or ceramic deposited on the top of the circuit protector over the previously deposited components or a polymer material applied to the top and cured by suitable means. Alternatively, the cover may comprise a plate of electrically insulating material, such as glass or ceramic, that is bonded to the top surface by suitable means, such as glass frit or adhesive. The cover may also comprise an uncured ceramic plate mechanically pressed on the top surface and cured to harden and bond it to the substrate.
The fuse element may be a deposited or printed film of gold or silver or another conductive material. Alternatively, the fuse element may comprise a conductive wire. Electrically conductive element pads may be provided to connect the fuse element with the termination pads and improve the electrical connection therethrough.
According to another aspect of the invention, a layer of thermally insulating material may be deposited on the top surface of the substrate below the termination pads and fuse element to improve the function of the fuse element by limiting heat transfer from the fuse element to the substrate.